The invention relates in particular to a system and a method combining a transmission of signals conveying any information and a measurement of distance between devices, in which the signals transmitted between the devices are digital signals, modulated, in particular by spread spectrum, and in which the measurement of distance is performed by estimating the propagation time of these signals between the devices.
The invention applies in particular to the transmission of data and measurement of distance between submarine devices communicating by acoustic waves.
Those skilled in the art are familiar with systems enabling distance to be measured between synchronized devices.
Among these, and generally, the measurement of distance between devices, including at least one sender and at least one receiver, is performed on the basis of the estimation of the propagation time of a monochromatic pulse or of a spread-spectrum modulated pulse sent by a sender, then received and time-stamped by a receiver.
The systems using a monochromatic pulse are well known to those skilled in the art and therefore will not be described below.
Document WO 96/03689 discloses a system in which the transmitted information is modulated by spread spectrum. The spread-spectrum modulation presents numerous advantages over a more conventional modulation because the signals modulated in this way provide for, among other things a far lower susceptibility to scrambling by signal interference, the sending of multiple signals over a frequency band and even the elimination or processing of multiple paths, multiple paths being better known by the name of fading.
In this document, the sole objective of the transmitted signals is to enable distance to be measured between devices. The means implemented are therefore provided to this end.
Furthermore, the spread-spectrum modulation in this case is more particularly performed according to a pseudo-random method. In this type of modulation, the signal sent is expressed in the form:a(t)=Ψ(t)*d(t) in which Ψ(t)=Φ(tmodts),t being the time, d(t) being a function taking positive and negative unit values, Φ(t) being a pulse sequence, including one or more bits, sent pseudo-randomly and of length ts, and Ψ(t) being a train of sequences Φ(t) repeated N times.
The estimation of the propagation time between two devices is performed on a correlator located in a receiver by time-stamping the passage of the pulse sequences by means of the peaks of the self-correlation function linked to these sequences. Then, the correlator performs a time-oriented integration of the product of the signal received and the various pulse sequences and deduces from this the time of arrival of the signal.
In this document, and, moreover, in numerous documents of the state of the art, a first pulse sequence is sent by a sender to a receiver which determines the propagation time, then this receiver of the first sequence becomes sender of a second sequence of pulses to the sender of the first sequence of pulses which then computes the propagation time of the second sequence of pulses. Knowing the propagation times of these two sequences, the distance between devices is then deduced, with increased accuracy. This method, well known to those skilled in the art, is particularly useful when the signal is of an electromagnetic type, given the high velocity of the signal involving short propagation times.
There are numerous applications for which the objective is not to measure a distance between devices but to convey any information, with specific means and according to a specific method.
In the case of communication between submarine devices, it is particularly interesting to be able to interchange any information such as, for example, information concerning the temperature, the pressure, the address of the sender or other information, and to establish the distance between devices based on the signal conveying the information.
Furthermore, in the case of submarine devices, the signals are transmitted by acoustic waves and the propagation times of these waves are very much longer than the times involved for electromagnetic waves. In this case, there is no point in performing one or more round trip measurements of a propagation time, but in this case it is essential to estimate as accurately as possible the propagation time of the signal between a sender and a receiver in order to deduce from this the distance separating them.